Neutrophils undergo a cell death pathway that results in release of chromatin coated with antimicrobial proteins forming a web called neutrophil extracellular traps (NETs). NETs trap and kill microbes. Our preliminary results show that NETs recruit and activate platelets. We have evidence of extracellular DNA/chromatin presence in pathological specimens from deep vein thrombosis and in plasma of patients with thrombotic disease. The central hypothesis of this application is that NETs provide a scaffold that, together with fibrin, increases thrombus stability and promotes thrombus growth and microparticle recruitment. We propose that the formation of NETs in pathological circumstances could initiate thrombotic events and contribute to conditions such as sepsis, thrombotic microangiopathies and deep vein thrombosis. In addition, we propose that DNase1, which digests NETs, may modulate thrombosis. We will analyze NETs in the context of in vitro and in vivo models of thrombosis. We will employ a flow chamber with video capability using cells from genetically engineered mice. We propose to study several mouse models of thrombosis, all of which are well established in our laboratory, to visualize NETs formation in vivo with sophisticated imaging technology and study the relevance of NETs to thrombus growth and stability. We are particularly keen to investigate the role of NETs in a new inferior vena cava stenosis model of deep vein thrombosis (DVT) and in a mouse model of sepsis. The proposal has 2 specific aims: 1. To study the molecular mechanisms mediating platelet and microparticle interactions with NETs. To evaluate whether NETs promote fibrin generation. In this Aim, we propose to investigate in vitro the molecular mechanisms involved in platelet adhesion to NETs and address whether NETs promote recruitment of pro-coagulant microparticles (MPs) and fibrin deposition. 2. To study NETs formation in vivo and examine the biological relevance of NETs to sepsis-induced thrombocytopenia and to deep vein thrombosis. To measure NETs biomarkers in samples from patients with thrombosis. In this aim, we propose to visualize NETs by intravital microscopy and by histology. We propose to modulate NETs production and circulating DNA levels and see the effects on murine models of sepsis and DVT. We will study NETs as biomarkers in thrombosis. Thrombosis, including deep vein thrombosis/pulmonary embolism is now the biggest killer in the United States. We hope that our studies will broaden understanding of the thrombotic process in general and that we may uncover new molecular targets to treat pathological thrombosis.